1. Field of the Invention
The present invention relates to the field of spin valve sensors.
2. Background Art
Spin valve sensors exploit changes in electrical resistance which occurs as a result of manipulating the relative orientation of the magnetization of ferromagnetic layers within a spin valve sensor. In conventional spin valve sensors, one ferromagnetic layer has its magnetization pinned while another, which has its magnetization set perpendicular to the pinned layer, is free to change its magnetic orientation in response to magnetized bits on an adjacent recording media. The magnetized bits on the recoding media, therefore, change the relative magnetization direction of the free layer with respect to the pinned layer. A current through the spin valve is used to detect changes in the resistance of the spin valve that results from changes in the relative magnetization of the free layer. Typically, the magnetic field of the free layer is set perpendicular to the magnetization of its pinned layer to provide a symmetrical operating point for the spin valve.
The sensing current, does itself create undesirable effects within the spin valve. The current through the spin valve generates resistive heat in the spin valve sensor. The heat can cause the instability and noise within the sensor.
Typically, an antiferromagnetic pinning layer, adjacent the pinned layer, pins the magnetic moment of the pinned layer. As the temperature of the pinning layer increases, the pinning field decreases. If the temperature of the sensor rises above the blocking temperature of the pinning layer, the pinning layer is no longer able to pin the magnetization of the pinned layer and may have its exchange coupling altered, causing the magnetization of the pinned layer to weaken, or change.
Turning to FIG. 1, another problem encountered with spin valves is Barkhausen noise. Barkhausen noise is caused as the magnetic field 301 and 302 from the magnetic media 210 passes the pinned layer 110. The magnetic field 301 and 302 from the media 210 causes domain walls 106 within the pinned layer 110 to shift, as illustrated in FIG. 1, temporarily changing the resistance of the pinned layer 110. Increased temperature and its corresponding reduction of the pinning field further intensifies Barkhausen noise by allowing the domain walls 116 to more easily move within the pinned layer 110.
Furthermore, the sensing current through the spin valve, particularly in the spacer layer between the pinned and free layers, induces a magnetic field which opposes the magnetic field of the pinned layer. This current induced magnetic field further reduces the strength of the pinned field and further contributes to Barkhausen noise. Moreover, this field can change the orientation of the pinned layer if the temperature of the pinned layer rises above its blocking temperature, thereby causing permanent damage to the spin valve.
Some of the above problems are also noted in U.S. Patent application Ser. No. 09/227,323, by Tong, et al., filed on Jan. 6, 1999, entitled SPIN VALVE SENSOR WITH IMPROVED PINNING STRUCTURE, issued as U.S. Pat. No. 6,185,077, on Feb. 6, 2001, herein incorporated by reference in its entirety.
Although the magnitude, or direction, of the current through the spin valve may be changed to improve the magnetization characteristic of the pinned layer and to reduce Barkenhausen noise, it would also undesirably change the operating point of the spin valve and lead to asymmetrical output.
The presently preferred embodiment of the spin valve of the present invention provides a magnetic compensation field within the spin valve. The compensation field extends or couples to the pinned layer so that it is in a reinforcing relationship with the magnetic moment of the pinned layer. The compensation field may be used to counteract any sensing current induced magnetic field, and its effects on the pinned layer.
The spin valve sensor of the present invention may be formed having a structure comprising: a free layer, a first spacer layer, a pinned layer, a pinning layer, a second spacer layer, and a compensation layer. The compensation layer may be formed of any known ferromagnetic material. The spacer layer may be formed of a nonmagnetic material of sufficient thickness to prevent pinning of the compensation layer by the pinning layer while providing a sufficiently small distance between compensation layer and the pinned layer to allow sufficient magnetic coupling to reduce the current induced magnetic field at the pinned layer.
The compensation field may be formed by setting the magnetic moment of the compensation layer during deposition, by depositing an adjacent layer having a set magnetization, by passing current through the compensation layer or through the spacer layer, or by any other techniques known in the art. The compensation field, however, is not derived from the pinning layer.
The spin valve sensor of the present invention may be utilized to provide an improved data storage and retrieval apparatus.
The present invention may be used to improve thermal stability and reduce Barkhausen noise while not impacting output symmetry.